1. Field of the Invention
The present invention relates to the improvements in a wafer holding member generally made of an aluminum nitride based sintered body, for example, a susceptor for holding a wafer, such as a semiconductor wafer or a glass substrate wafer for liquid crystal display. More particularly, the inventions enable quick heating of a wafer holding member mainly made of an aluminum nitride based sintered body in which various electrodes and heating resistors are embedded and to the invention for improving corrosion resistance against halogenic corrosive gases.
2. Prior Art
In a semiconductor production process, a wafer holding member, such as a susceptor or an electrostatic chuck, is used to hold a wafer in a processing chamber of a CVD apparatus for forming a thin film on a semiconductor wafer, a dry etching apparatus for micromachining the wafer, or the like.
As shown in FIG. 1, a susceptor 10 holds a wafer 30 placed on a disc-shaped base body 11 by pressing the wafer 30 with a clamp ring (not shown). A heating resistor 16 for heating the wafer 30 can be embedded in the base body 11. In addition, as shown in FIG. 2, an electrostatic chuck 20 is provided with a single electrode or a plurality of electrodes (a plasma generation electrode, an electrostatic adhesion electrode, etc.; only the plasma generation electrode 23 is shown in FIG. 2) embedded in a base body 21. By applying DC voltage 25 to the electrode 23, electrostatic adhesion force is generated so as to secure the wafer 30 by adhesion on the surface of the chuck 20.
In addition to holding a semiconductor wafer, this kind of wafer holding member is also used to hold a glass substrate wafer for liquid crystal display or the like.
Ceramics, such as alumina ceramics, has been used as a material for the base body of the susceptor 10 or the electrostatic chuck 20. However, in these days, a proposition has been made to use aluminum nitride ceramics having high thermal conductivity and high corrosion resistance against halogenic corrosive gases (refer to Japanese Laid-open Patent Application No. 6-151332). In this case, tungsten (W) or molybdenum (Mo) is used as a conductive material for the heating resistors 16, 26, electrostatic adhesion electrode and plasma generation electrode 23 embedded in the base bodies 11, 21. In general, a paste of this kind of metal is printed in a predetermined pattern on green sheets made of aluminum nitride and the sheets are laminated and fired integrally.
However, in the production process of the wafer holding member, wherein a conductive material, such as W or Mo, is embedded as the heating resistors 16, 26 or the like in the base bodies 11, 21 made of aluminum nitride ceramics, the base bodies 11, 21 are cracked or the heating resistors 16, 26 made of such a conductive material are separated or disconnected because of the difference in contraction coefficient between the base bodies 11, 21 and the conductive material portions 16, 26 during firing. In particular, when high-purity aluminum nitride ceramics is used for the base bodies 11, 21, the above-mentioned problems due to the difference in contraction coefficient occur significantly.
On the other hand, when the obtained wafer holding member is used, since the thermal expansion coefficient of the aluminum nitride constituting the base bodies 11, 21 is 5.times.10.sup.-6 /.degree. C., and the thermal expansion coefficients of W and Mo used as conductive materials are 4.6 to 4.8.times.10.sup.-6 /.degree. C. and 5.7.times.10.sup.-6 /.degree. C. respectively, cracks and other problems are apt to occur because of the difference in thermal expansion coefficient. For example, in the case of the susceptor 10 in which the heating resistor 16 is embedded, since the ON-OFF pulse control of a voltage of 100 V or more is performed, quick heating occurs during operation. Cracks are thus caused at the interface between the base body 11 and the heating resistor 16, or the heating resistor 16 is separated or disconnected because of the above-mentioned difference in thermal expansion coefficient. Therefore, the susceptor 10 cannot be heated quickly, causing problems of significantly reducing operation efficiency and requiring temperature control during heating.
In the cases of the electrostatic adhesion electrode and the plasma generation electrode 23, quick heating is not necessary. However, cracks or other problems occur in the base bodies 11, 21 during usage for an extended period of time because of the difference in thermal expansion coefficient between the electrodes and the base bodies 11, 21, since the electrodes are large in size.
Furthermore, in these wafer holding members, heating speed, heating temperature, etc. during heating by energizing the heating resistors 16, 26 are controlled in accordance with the following methods:
(1) Temperature is measured by using a thermocouple, and voltage to be applied is changed depending on the measured temperature. PA1 (2) A constant voltage of 120 V or the like is turned on and off repeatedly to control heating speed and heating temperature. PA1 (3) A constant voltage of 120 V or the like is applied for a certain period and then turned on and off repeatedly to control heating temperature.
Among the above-mentioned power application methods for the wafer holding member, the method (1) for adjusting the voltage requires difficult control. Therefore, the method (2) or (3), wherein a constant voltage is turned on and off repeatedly to perform control (referred to as "PID control"), is used usually.
In addition, since such a wafer holding member requires a large output in consideration of heat dissipation due to radiation and conduction and is usually used at a high temperature of 300.degree. C. or more, the heating resistors 16, 26 are designed to have predetermined resistance values in their operating temperature ranges.
However, since the resistance values of the conductive materials constituting the heating resistors 16, 26 generally increase as the temperature rises, the resistance values of the heating resistors 16, 26 at room temperature are lower than the predetermined values. For this reason, when a constant voltage is applied in accordance with the above-mentioned method (2) or (3), there is a danger of causing overcurrent at room temperature.
When 1500 W is required at 500.degree. C. by using a 120 V AC power supply 14 or 24, the resistance value of the heating resistor 16 or 26 is 9.6 .OMEGA., and a current of 12.5 A flows. In case the heating resistor 16 or 26 is made of WC, its resistance value at 500.degree. C. is nearly three times as high as the resistance value at room temperature. Accordingly, the resistance value at room temperature is 3.2 .OMEGA.. When a voltage of 120 V is applied, a current of 37.5 A flows. When an overcurrent of 20 A or more flows in this way, a thermal stress due to abrupt temperature change occurs, thereby causing cracks in the base bodies 11, 21 and breaking lead wires and terminals for energizing the heating resistors 16, 26.
These are problems caused during production and usage because of a substantial difference in thermal expansion coefficient between the base body and the energized portions (heating resistors and electrodes). In particular, special attention should be paid to the fact that the wafer holding member cannot be heated abruptly.
An aluminum nitride based sintered body is used for the wafer because of the following reasons: As described earlier, the aluminum nitride based sintered body is superior in heat resistance and thermal shock resistance, and hardly corroded by halogenic corrosive gases, and yet has high thermal conductivity. The aluminum nitride based sintered body contains a sintering aid, such as a rare-earth oxide, Ni compound, rare-earth fluoride or fluoride, to have higher thermal conductivity.
However, the above-mentioned aluminum nitride based sintered body contains about 97 wt % of AlN; the sintered body does not have high purity. Furthermore, since the sintered body contains a sintering aid, numerous grain boundary phases are present in the sintered body. When this corrosion-resistant member made of the sintered body is used in a plasma-generated halogenic corrosive gas atmosphere, the grain boundary phases are etched and aluminum nitride particles are separated. As a result, sufficient corrosion resistance is not obtained.
Besides, when a wafer, such as a semiconductor wafer or a glass substrate wafer for liquid crystal display, is subjected to film forming or micromachining by using the wafer holding member made of an aluminum nitride based sintered body in a plasma-generated halogenic corrosive gas atmosphere, the wafer mounting surface made of the aluminum nitride based sintered body is etched and particles are generated, causing the problem of adversely affecting the traces or the like on the wafer.
Moreover, since the aluminum nitride based sintered body constituting the wafer holding member contains large amounts of a sintering aid and impurities, such as Na, Ca and Fe, there is a fear of contaminating the wafer.
These problems are raised because the conventional aluminum nitride based sintered body has insufficient corrosion resistance against halogenic corrosive gases.